1. Field of the Invention
The present invention relates to a solenoid device having a plunger and a solenoid to displace the plunger, an automatic document feeder having the solenoid device configured to move a member capable of transferring or guiding a document, and an image forming apparatus having the automatic document feeder.
2. Description of the Related Art
Patent Document 1 discloses a solenoid device having a plunger biased toward a maximum extension position (position that the plunger extends furthest from a solenoid side) by a coil spring and a solenoid which displaces the plunger. When the plunger attracted in the solenoid is repelled back to the maximum extension position, a time to supply current pulses to the solenoid is shortened so that a displacement speed of the plunger is reduced, instead of suddenly stopping a current supply to the solenoid. In this manner, an impact noise made when the plunger is repelled back to the maximum extension position is reduced.
Patent Document 2 discloses a solenoid device having a plunger biased toward a maximum extension position by a coil spring and a solenoid which displaces the plunger. When displacing the plunger by supplying a current to the solenoid, the current is supplied to the solenoid plural times. In this manner, an impact noise made when the plunger is attracted to a maximum attraction position (position where the plunger is attracted closest to the solenoid) is reduced.
[Patent Document 1] Japanese Patent Application Publication No. 10-139179
[Patent Document 2] Japanese Patent No. 3561679
In the technique disclosed in Patent Document 1, however, an electric circuit control element such as a pulsed current supply driver is used to control the current supply. Therefore, a circuit device may become complicated and the manufacturing cost may be increased. In the technique disclosed in Patent Document 2, the impact noise cannot be sufficiently reduced if an operation environment changes in such a manner that an attraction force of the solenoid decreases due to a raised temperature of the solenoid, or a mechanical load on the plunger varies.